On 9 Sep 2006 17:28:03 -0700, in message , "Radium" scribed:
Sit down, calm down, find a reference book, and read along with the discussion.
Answer a simple question; are you asking about theoretical possibilities or administrative possibilities?
You need to develop a discriminating mind (pun somewhat intended).
Modulation here means multiplication of the signal (plus a DC offset) by a sinusoidal carrier. That's what AM is.
Out = ( sig(t)+1 ) * sin(2*pi*150000*t)
where sig(t) is the signal.
Right, a cheapo envelope detector doesn't work well with super-wideband AM, but there are several simple ways to perfectly recover the signal, a carrier-phase-locked lo and another multiplier, followed by a good lowpass filter, being one. I think an ideal diode rectifier or a square-law detector, either followed by a lowpass, work too.
As long as Fm < Fc, the spectrum is unambiguous, so perfect demodulation must be possible.
It gets interesting if the receiver has a front-end highpass filter that passes nothing below the carrier frequency. The result is sort of a VSB signal (actually, it's SSB+carrier) and all the low-frequency aliases are discarded. In that case, I think you can use a simple AM modulator up to Fm = 300 KHz, twice the carrier frequancy!
John
What boat did I miss? Why would only the peaks of the carrier sinewave "sample" the signal, and not the entire sinewave? And how does your theory deal with the fact that the sine-carrier "samples" alternate in polarity, whereas a real 2F sampler wouldn't? They are nothing like equivalent.
Look at the spectrum of a sine wave, and look at the spectrum of an impulse train. They are very different, and, specifically, a sine wave has no harmonic lines, and an impulse train has all of them, including the DC term. That's why a linear lowpass filter recovers PAM signals but can't recover AM.
John
Well, if people state things as facts and speak with authority, they might consider making some modest effort to be right. It doesn't help beginners to tell them stuff that's nonsense.
John
Better yet, realize that the process is time-continuous so there are an infinite number of "samples" per cycle.
John
Eureka, that's it: the fact that the sinewave carrier "samples" alternate in polarity means the baseband component of an AM modulated signal is zero, as contrasted to a truly sampled signal where the baseband spectrum is preserved intact.
Cool stuff.
John
What is the bandwidth of the receiver? That is the limiting factor.
What are you attempting to do?
>
That still depends on the bandwidth of the receiver. It's a receiver design issue. Is this something you intend to build or buy?
Theoretical
150 Khz
Find out the highest possible frequency of modulation that a 150 khz AM station can transmit or receive.
What is the lowest physically-possible frequency of an AM station that will allow me to hear a 20,000 khz tone on the receiver?
Surely a 1 hz station wouldn't work for this [DUH!!!!]
I am simply looking for an scientific answer to a question that interests me.
In a previous post you said the bandwidth of the receiver is 150 kHz. Now you say the carrier frequency of the signal is 150 kHz, So we can assume the bandwidth extends to 75 kHz above and below the carrier. Is this what you assume for the receiver? If not then make your assumption set more explicit.
Are you assuming a standard double sideband signal, or a single sideband signal?
OK, on reconsideration, an envelope or square-law type detector won't work to properly demodulate super-wideband AM, because it will generate too many spurious cross-products. The synchronous detector is still OK.
John
To quote our new "scientist" poster..... Well Duh!
Actually AM, as assumed by the poster, is not typically "demodulated," it is "detected." Not unlike charging a capacitor from PAM pulses.
Your question is totally unclear to me. To what signal do you refer?
Don
That's two of us in the Northwest... The land of some great microbreweries.
Don
You imply here that the carrier has bandwidth; it doesn't. I assume you meant to say "...modulating frequency exceeds the frequency of the carrier,...."
Okay. Let me make a modification. Lets make the the the bandwidth of the receiver 150 Ghz [notice that 'G'] but keep the carrier frequency of 150 Khz. Now, what is the maximum frequency the the modulation can be? I guess its 750 Mhz. Do I guess correctly?
Standard.
If the carrier frequency is 1 hz but the bandwidth of the reciever is
40 Khz, then could I [at least in theory] hear anything on the speaker of the receiver?
You are establishing unrealistic conditions.
I suggest you use Google to do some homework. Search "amplitude modulation," which is a multiplication process having application, not only in transmitter modulation per se', but also in designs of transmitters and receivers where it is desirable to move a single frequency or band of frequencies to a different frequency range (intermediate frequency). Particularly look a the modulation product algebra.
How is it unrealistic?
Obviously, it is not only the peaks which "sample" the signal, but you CAN use sampling theory in an analysis of AM if you treat it from that perspective. It's hardly "my" theory.
Bob M.
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